refactor(fe): query-key factory + migrate feed/channels/playlists (R7 S2a)

Introduce lib/queryKeys.ts as the single source of truth for React Query
keys — the R7 driver was ~200 hand-written key literals across 51 files
(`["feed"]` alone in 15+), where one typo silently orphans a key so an
invalidate never matches.

S2a migrates the three highest-traffic domains (feed/video, channels,
playlists) plus the shared me/my-status/tags roots: ~90 literal sites in
18 files now call qk.*(). Pure substitution — each factory fn returns the
byte-identical array the call sites used before, so TanStack's prefix
matching is unchanged (qk.feed() still invalidates qk.feed(filters)).
Co-invalidation clusters are deliberately NOT bundled into helpers (they
differ per site), so no query's refetch scope changes.

Nullable keys (playlist(selectedId), ytSearch(q)) accept null so a null
segment is preserved (["playlist", null]) while a no-arg call is the bare
prefix key — guard is `!== undefined`.

Gate green: typecheck, eslint (0 err), prettier, 68 vitest. Smoke: app
boots, feed/facets/tags/my-status all 200, no console errors. Factory
extended to plex/messaging/admin/downloads/notifications in S2b.

frontend/src/components/Playlists.tsx

@@ -1,6 +1,7 @@
 import { lazy, Suspense, useCallback, useEffect, useMemo, useRef, useState } from "react";
 import { useTranslation } from "react-i18next";
 import { useQuery, useQueryClient } from "@tanstack/react-query";
+import { qk } from "../lib/queryKeys";
 import {
   DndContext,
   KeyboardSensor,
@@ -171,8 +172,8 @@ export default function Playlists() {
           // Refresh the sidebar (cover may change) and the detail (so the now-dirty state,
           // and the Reset/Unsynced indicators, show without an F5). The membership guard
           // keeps the refetch from wiping the undo history on a pure reorder.
-          qc.invalidateQueries({ queryKey: ["playlists"] });
-          qc.invalidateQueries({ queryKey: ["playlist", selectedId] });
+          qc.invalidateQueries({ queryKey: qk.playlists() });
+          qc.invalidateQueries({ queryKey: qk.playlist(selectedId) });
         })
         .catch((e) => {
           // Was a bare .then(): a failed reorder left the UI showing an order the server had
@@ -182,7 +183,7 @@ export default function Playlists() {
           // the arriving detail is judged "same list" and the rejected order stays on screen — and
           // becomes the base for the next drag.
           lastSetRef.current = "";
-          qc.invalidateQueries({ queryKey: ["playlist", selectedId] });
+          qc.invalidateQueries({ queryKey: qk.playlist(selectedId) });
         });
     },
   });
@@ -201,14 +202,14 @@ export default function Playlists() {
   const [playingId, setPlayingId] = useState<string | null>(null);
 
   const listQuery = useQuery({
-    queryKey: ["playlists"],
+    queryKey: qk.playlists(),
     queryFn: () => api.playlists(),
     refetchOnWindowFocus: true,
   });
   const playlists = listQuery.data ?? [];
 
   const detailQuery = useQuery({
-    queryKey: ["playlist", selectedId],
+    queryKey: qk.playlist(selectedId),
     queryFn: () => api.playlist(selectedId as number),
     enabled: selectedId != null,
     // Refetch when returning to the tab so a change made elsewhere (another tab/device)
@@ -331,8 +332,8 @@ export default function Playlists() {
   const plName = (p: { kind: string; name: string }) => playlistName(p, t);
 
   function refreshAll() {
-    qc.invalidateQueries({ queryKey: ["playlists"] });
-    qc.invalidateQueries({ queryKey: ["playlist", selectedId] });
+    qc.invalidateQueries({ queryKey: qk.playlists() });
+    qc.invalidateQueries({ queryKey: qk.playlist(selectedId) });
   }
 
   async function revertToYoutube() {
@@ -499,14 +500,14 @@ export default function Playlists() {
           (onYoutube ? t("playlists.deleteYtFailed") : t("playlists.deleteFailed")),
       });
     }
-    qc.removeQueries({ queryKey: ["playlist", deletedId] });
-    qc.invalidateQueries({ queryKey: ["playlists"] });
+    qc.removeQueries({ queryKey: qk.playlist(deletedId) });
+    qc.invalidateQueries({ queryKey: qk.playlists() });
   }
 
   function playerState(id: string, status: string) {
     api.setState(id, status).then(() => {
-      qc.invalidateQueries({ queryKey: ["playlist", selectedId] });
-      qc.invalidateQueries({ queryKey: ["feed"] });
+      qc.invalidateQueries({ queryKey: qk.playlist(selectedId) });
+      qc.invalidateQueries({ queryKey: qk.feed() });
     });
   }